The term “antenna array” refers to a geometrical arrangement of a number of antenna elements. The antenna elements can be configured as a single antenna unit to achieve a desired antenna gain and directional characteristics, such as a particular radiation pattern. A variation of this radiation pattern can be referred to as beam-forming. Antenna arrays can have applications, such as in multiple-input multiple-output (MIMO) communication systems. In particular, very large antenna arrays can be referred to as “massive MIMO arrays”. Massive MIMO arrays may use several hundreds of antenna elements arranged in a single antenna unit and are considered to be a key technology component for future communication systems, such as fifth generation, 5G, communication. According to known technology, an up-link MIMO unit may comprise for example a radio base-station receiver, an analog-to-digital converter and automatic gain control units.
Massive MIMO can have some advantages, however, these advantages are counteracted, in practice, by an increased hardware complexity associated with having many antennas and many associated up/down conversion chains, and by an increased energy consumption due to all the hardware required for operation.
U.S. Pat. No. 9,705,579 provides a method with a complexity of using a MIMO system, while retaining some benefits as antenna selection, where a subset of size L taken from a set of N available antenna signals is selected and connected, via a switch, to L (L<N) radio-frequency (RF) chains. However, this method fails short in providing an amount of beamforming gain, and thus shows reduced or unacceptable performance, in particular, in channels with small angular spread, which typically occurs in conventional cellular systems.
In wireless communications, channel state information (CSI) refers to known channel properties of a communication link. This information describes how a signal propagates from the transmitter to the receiver and represents the combined effect of, for example, scattering, fading, and power decay with distance. The CSI makes it possible to adapt transmissions to current channel conditions for achieving reliable communication with high data rates in multi-antenna systems.
Most, conventional CSI estimation methods developed for traditional MIMO systems have a significantly more number of transmitters and receivers used at the base station, and are not always suitable for massive MIMO channel estimation due to the. The number of transceivers at the base station can potentially reach to thousands when the carrier frequency moves up to the millimeter wave frequency band which makes the physical size of the antennas much smaller, e.g., comparable to the wavelength.